Common methods of operation in laser cutting machines for sheet materials perform two-dimensional (2D) relative motion between a laser and a workpiece, such that a laser beam cuts the workpiece as the beam moves. In such machines, there are three options. A position of the laser is fixed and the workpiece is moved in X and Y directions. A position of the workpiece is fixed and the laser is moved in the X and the Y directions. The laser is fixed in the Y direction and moves in the X direction, and the workpiece is fixed in X direction and moves in Y direction.
For example, a laser tube cutter fixes the position of the laser, and the position of the workpiece is translated and rotated to a focus point of the laser. In contrast, large CO2 laser cutters used for cutting steel plate leaves the position of the workpiece stationary, and move the laser. A machine of the third type, such as a sign cutter, which needs to operate on very long lengths of material, spools the flexible material in the Y direction, while moving the laser only in the X direction.
Each type of machine has advantages and disadvantages, most of which are based on the final production rate and work material characteristics. To a significant extent, the ultimate speed of the laser cutting machine is determined by inertia of the parts of the machine that have to move in order to make the cut.
For example, in one laser cutting machine, the weight of a lens carriage is on the order of 50 Kg, riding on a Y-axis weighing on the order of 200 Kg, while the workpiece itself may weigh as much as 1000 Kg. In this case, it makes sense to move the lens carriage in both the X and Y directions as the lens carriage has inertia lower than the workpiece. In such laser cutting machine, the lens carriage is equipped with diagonal transfer mirrors, or an optical fiber to transfers the laser beam from the laser power oscillator to the lens carriage.
A limiting factor on production rate is directly related to the inertia of the components of the laser cutting machine. Therefore, reduction of the effective inertia of the components has a direct impact on the productivity of the laser cutting machine.
For example, one laser cutting machine uses a pair of redundant axes of the motion, i.e., a planar gantry with a high inertia, and a polar gantry with a low inertia. However, the inertia of the laser even in this machine is still relatively large, as the laser focusing lens itself is moved. Such motion also stresses the precision optics of the laser focusing lens and may lead to a suboptimal cut.
Another possible method of reducing inertia is to replace the X-Y motion completely with a pair of mirrors moved by galvano drives. Each galvano drive and mirror assembly provides one axis of deflection to a laser beam. By using two such galvano drives at right angles, a rectilinear area can be scanned. To simplify this assembly for system builders, Nutfield and Edmund Industrial Optics also sell pre-packaged XY galvano system “scan heads” in both open-frame and enclosed form factors for use as black-box components in constructing video and laser systems. With a suitable choice of lenses and lasers, these XY galvano scan heads can be used as beam directors, or as laser engraving devices.
In one exemplar laser drilling machine, the workpiece is mounted onto a worktable and fixed in position. The worktable is then traversed in the X and Y directions, and a pair of video cameras is used to precisely locate the workpiece with respect to the machine frame and the laser lens. A position of the workpiece table is locked, and the laser beam is directed solely by motion of two galvano drives rotating a pair of mirrors arranged to deflect the beam in both the X and Y direction. The drilling laser beam then travels through an F-theta field-flattening lens so that the beam is focused to under 100 microns diameter at any point on the workpiece surface, despite the change in distance due to the beam moving along the diagonal. Because the galvano drive and F-theta lens have only a limited rotation and limited beam aperture (approximately +/−15 degrees mechanical for the galvanos and 25 mm aperture for the F-theta lens), the workpiece accessible area is limited to roughly 50 mm×50 mm (about a 2″ square). After all laser drilling desired in the 50 mm×50 mm area is completed, the traversing table is unlocked and the traversing motors are used to move the workpiece to the next work zone, where the workpiece is again precisely located via video cameras, locked, and the galvano is used for the cutting.
In another machine, the XY galvano scanhead is placed on the end of a multi-axial industrial robot arm. The path of the robot end is then constrained to stay within a “mobility tube” describing the set of positions where the galvano scanhead is capable of aiming at the area to be machined. The correct set of multi-axial robot joint motions, combined with the proper galvano drive signals is dynamically determined by the control unit. However, avoidance of “robot arm crashes” in this design, i.e., situations where the robot arm tries to pass through itself or through the workpiece, is computationally difficult, and requires not only an accurate model of the robot arm and scanhead, but also a continuously evolving model of the workpiece during different stages of the cutting process, because a forbidden motion at one stage in the cutting process may be available for another stage of the process.
Accordingly, there is a need in the art to address disadvantages described above.